Directly heated cathode



Patented Aug. 2, 1 949 UNITED srrras Aram* DIRECTLY HEATED CATHODE Application June 11, 1947, Serial No. 753,862

This invention relates to cathodes for electrondischarge devices, and more particularly to cathodes for electron-discharge devices of the magnetron type.

Modern, relatively high-power, electron-discharge devices of the general character above referred to usually employ indirectly-heated cathodes and, for certain purposes, this has been found to be disadvantageous.

It is, therefore, the main object of the present invention to provide a directly-heated cathode of high electron emissivity which may be used advantageously, but not exclusively, in electrondischarge devices of the magnetron type.

In the accompanying specication there shall be described, and in the annexed drawing shown, an illustrative embodiment of the directly-heated cathodes of the present invention. It is, however, to be clearly understood that the present invention is not to be limited to the details herein shown and described for purposes of illustration only, inasmuch as `changes therein may be made without the exercise of invention, and within the true spirit and scope of the claims hereto appended.

In said drawing,

Fig. 1 is a fragmentary, longitudinal sectional view of a cathode made in accordance with the present invention;

Fig. 2 is a transverse sectional view of the same taken on line 2-2 of Fig. 1;

Fig. 3 is a longitudinal sectional view of a mold which may be utilized for fabricating the cathodes of the present invention; and

Fig. 4 is a transverse sectional view of the same taken on line 4-4 of Fig. 3.

Referring now more in detail to the aforesaid illustrative embodiment of the present invention, with particular reference to Figs. 1 and 2 of the drawing, the numeral l generally designates a cathode which includes an outer tubular conductor l l and an inner conducting rod l2 coaxial with said outer conductor.

The rod I2 extends beyond the conductor ll and is threaded at its outer end, as at I3, to engage a conducting cap I4 which is thereby held in spaced relationship to the end of said conductor Il.

The opposed edges of the conductor l l and the cap lll may be shouldered, as at l5 and It, to support therebetween an electron-emissive body I 'l in the form, for example, of a cylindrical sleeve.

The body I1 preferably consists of compressed, intimately admixed, powdered thoria (thorium 4 Claims. (Cl. Z50-27.5)

Y 2 y oxide) and a powdered refractory metal, metal compound or metal alloy, such as-molybdenum, molybdenum carbide, mo1ybdenumcoba1t,molyb 2 denum-nickel, tungsten, tungsten carbide, tungsten-cobalt, or tungsten-nickel. Preferably, the

refractory metal employed is molybdenum and,;

in this case, the` molybdenum should constitute from about 16'to about 30%, by Weight, of the mixture. With lesser amounts of metal, the body is brittle, while with larger amounts of metal, the body passes too much current. Between 20 and 25% of molybdenum has been found to give quite good results.

When a suitable voltage is applied between the conductors I'I and l 2, the current passing through the body Il heats said body to a temperature of thermionic emission, the structure thereby constituting a directly-heated cathode which has been found particularly suitable for use in electron-discharge devices of the magnetron type.

The body I1 may be fabricated in the following manner.

In Figs. 3 and 4 of the drawing, there is shown a mold, generally designated by the reference character 20, resting upon a support 2|. The mold includes a cylindrical body 22 provided with a tapered bore 23 receptive of a sectional sleeve 24, the outer surface of which closely follows the contour of said bore. The sleeve 24, when assembled, defmes a cylindrical bore 25 receptive of a plurality of tubular plungers 2B, 2'! and 28, the latter, in turn, being receptive of a central pin 29, the upper end of which functions to guide a cylindrical ram 30 by means of which pressure may be applied to the uppermost plunger 28.

In utilizing the mold 20, the sections of the sleeve 24, here shown as three in number, are assembled and fitted into the bore 23 of the body 22 under pressure. Thereafter, the plunger 26 is mounted on the lower end of the pin 29, said plunger and pin then being inserted into the bore 25 dened by the sleeve 24. An annular space is thus provided between the outer surface of the pin 29 and the bore 25 of the sleeve 24. Into this space a quantity of thoroughly admixed, powdered thoria and a metal, such as molybdenum, here designated by the reference character M, is poured, after which the plungers 21 and 28 are slid onto the pin 29 as is the ram 30.

With the lower surfaces of the sleeve 24, the pin 29 and plunger 2B rmly resting on the support 2|, about 250,000 pounds per square inch of pressure is exerted against the upper surface of the ram 30, with the result that the mixture M between the plungers 26 and 21 is rmly compacted.

After relieving the pressure and removing the sleeve 24 from the body 22, said sleeve is dismembered to permit the electron-emissive body Il formed from the mixture M to be removed from the pin 29. At this stage the cathode is green and itmust be red to give it mechanical strengthyand electrical conductance. For this purpose, it is heated to a temperature of from about 1300 to about 2200 C. for from about 5 minutes to about 3 hours, depending upon the characteristics desired. Too low a-temperature and too short a time of firing resultiinpoor con-` trol of dimensions and resistanceywhile toohigh a temperature and too long a firing period result in excessive evaporation of volatile metals.

This completes the description *of the aforesaid illustrative embodiment of the present in-y What is claimed is:

1. A directly-heated cathode comprising a molded sleeve of powdered thoria and molybdenum. Y

2. A directly-heated cathode comprising a cylindrical sleeve of powdered thoria and molybdenum.

3. A directly-heated cathode corriprisingv a compressed mixture of vpowdered thoria andmolybdenum, said molybdenum constituting from about l6'to about 30%, by weight, of said mixture.

4. A directly-,heated cathode comprising a mixture of powdered thoria and molybdenum compressed to form'a cylindrical sleeve, said molybdenum constitutingfrom about 16 to about 30%, by Weight, :of :said mixture.

JOHN F. HANSON.

REFERENCES CITED The Vfollowing referencesfare ofrecord in the le 01E-this patent;

vUNITED vS'I'lLIElS PATENTS Number Name Date 1,663,553 Iredell ,Mar. 27, 1928 2,208,920 Allen July 23, 1940 

